MRI imaging has become a widely-used and well-known imaging modality for generating images of interior portions of the human body. Because those of ordinary skill in the art are quite familiar with the basic concepts of MRI, those concepts need only be briefly set forth as background for the invention.
Toward that end, as is well-known, MRI machines are used to create images of interior portions of the body. In doing so, an MRI machine applies a magnetic field to at least a portion of the body to be imaged. A typical magnetic field strength is 1.5 T, although other field strengths are used (commonly in the range of 0.5 T–3.0 T). Thereafter, localized gradients are created in the magnetic field, and RF pulses are applied to a target area representing the portion of the body for which an image is desired. A typical frequency for the RF pulse is the Larmour frequency (around 63 MHz for protons in a magnetic field of 1.5 T). Protons in the target area absorb energy from the RF pulse in an amount sufficient to change their spin direction. Once the RF pulse is turned off, the protons release excess stored energy as they return to their natural alignment in the magnetic field. When releasing this stored energy, signals are created that are indicative of an image of the target area. When properly sensed, such signals can be processed by a computer to generate an MR image of the target area.
It is known in the art to receive such signals through the use of an intracorporeal RF probe (also referred to as an RF receiver). When disposed in the body proximate to the target area, such RF probes are capable of sensing the proton emissions and providing the sensed signal to the image generating computer system by way of a transmission medium such as a coaxial cable. Because such probes may be inserted into the body through very small openings, it is important that those receivers have as small of a mechanical envelope as possible.
Also, it is important that the receiver coil resonate (i.e., efficiently store energy) at the Larmour frequency. To resonate a particular frequency f, the inductive components (L) and capacitive components (C) of the receiver coil should satisfy the following equation:
  f  =      1          2      ⁢      π      ⁢              LC            
The RF probes in prevalent use for MR imaging can be grouped into two basic categories (1) an elongated coil with a thin cross section, and (2) a loopless antenna (dipole) consisting of a single thin wire. An example of an elongated coil design for an RF receiver is described by Quick et al. in Single-Loop Coil Concepts for Intravascular Magnetic Resonance Imaging, Magnetic Resonance in Medicine, vol. 41, pp. 751–758 (1999), the entire disclosure of which is hereby incorporated by reference. An example of a loopless antenna design is described by Ocali and Atalar in Intravascular Magnetic Resonance Imaging Using a Loopless Catheter Antenna, Magnetic Resonance in Imaging, vol. 37, pp. 112–118 (1997), the entire disclosure of which is hereby incorporated by reference. Other coil examples are Helmholtz coils (which typically consist of two single loop coils in parallel) and flat coils.
FIG. 1 illustrates an exemplary prior art coil receiver assembly. A single loop coil 100 senses the signal emitted by the target area responsive to the RF pulses. Both coil 100 and thin coaxial cable 102 can be disposed inside the body of the patient. The signal passes from coil 100 through thin coaxial cable 102 to thicker coaxial cable 104, which may be RG 58 cable or the like. Together the thin and thick coaxial cables 102 and 104 have a length of λ/2 and form part of a tuned resonance circuit. The coil receiver assembly also includes an external tuning/matching circuit 106 as shown, wherein variable tuning capacitor Ct forms a resonant circuit with the inductance of the coil 100 and cables 102 and 104, and variable matching capacitor Cm matches the input impedance of the resonance circuit with that of the receiver (50 Ω).
FIGS. 2(a) and 2(b) illustrate an exemplary prior art antenna receiver assembly. Dipole antenna 110 is shown in FIG. 2(a). The dipole antenna 110 is formed of two separated conductors 112 and 114. As the current path is not complete, charge oscillates between the two tips of the conductors 112 and 114. When implemented, the antenna 110 is coupled with thin coaxial cable and disposed within a catheter 120. Catheter 120 may be inserted within the body proximate to the target area for imaging thereof. For satisfactory quality of performance, the input impedance of the antenna 110 (ZIN) must be matched with the characteristic impedance of coaxial cable 122 shown in FIG. 2(b). Also, to avoid interference caused by antenna resonation, detuning is needed to electronically damp the receiver's resonance by presenting the coaxial cable to the antenna as a large magnitude impedance. For these purposes, external tuning/matching/decoupling circuit 124 is provided to link the catheter 120 with coaxial cable 122 (which itself terminates at connector 126).
Such prior art receiver assemblies suffer from various shortcomings, namely (1) the single loop coil design exemplified by FIG. 1 works well for near field resolution but not for far field resolution (due to field cancellation occurring at a relatively short distance from the loop)—the near field and far field pertaining to the physical location of the imaging field relative to the receiver, (2) the antenna design exemplified by FIGS. 2(a) and 2(b) works well for far field resolution but not for near field resolution (as determined by the device's geometry which defines a near/far transition zone), (3) each design requires the use of bulky and relatively expensive external matching circuits and tuning circuits, and (4) the coil design of FIG. 1 allows heat to build up as current passes through the coil. While Helmholtz coils and flat coils do not suffer from troubling near/far field transition zones, those coils require the use of external matching and tuning circuits.
Additional coil designs are shown in the article Rivas et al., “In Vivo Real-Time Intravascular MRI”, Journal of Cardiovascular Magnetic Resonance, 4 (2), pp. 223–232, 2002 (the entire disclosure of which is hereby incorporated by reference), all of which suffer from the same or similar shortcomings mentioned above.
Therefore, there is a need in the art of medical imaging for an RF probe that provides high performance in both near field and the far field imaging. Further, there is a need in the art of medical imaging for an RF probe that avoids the incorporation of bulky external electrical components such as matching circuits and tuning circuits which not only adversely affect the size of its mechanical envelope but also add to the cost of the receiver.